granular anchors under vertical loading/axial pull
TRANSCRIPT
1
Granular Anchors Under Vertical Loading/Axial Pull 1
2
3
4
by 5
6
V. Sivakumar1, B.C. O’Kelly
2, M.R. Madhav
3, C. Moorhead
4 and B. Rankin
5 7
8
9
10
11
12
13
Revised paper submitted to CGJ for consideration for publication 14
15
16 1Corresponding author: 17
V. Sivakumar 18
School of Planning, Architecture and Civil Engineering 19
Queen’s University Belfast 20
BT7 1NN 21
23 2Trinity College Dublin 24
Department of Civil, Structural and Environmental Engineering 25
Museum Building 26
Dublin 2, Ireland 27
29 3J.N.Technical University 30
159 Road No. 10 Banjara Hills 31
Hyderabad 500034 32
India 33
35 4QUB - Civil Engineering 36
Belfast, NI 37
United Kingdom 38
40 5QUB - Civil Engineering 41
Belfast, NI 42
United Kingdom 43
45
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Granular Anchors Under Vertical Loading/Axial Pull 46
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V. Sivakumar, B. C. O’Kelly, M.R. Madhav, C. Moorhead and B. Rankin 48
49
50
Abstract 51
52
Granular anchors are a relatively new concept in ground engineering with relatively 53
little known regarding their load–displacement behaviour, failure modes, ultimate 54
pullout capacity and also potential applications. A granular anchor consists of three 55
main components: a base plate; tendon and compacted granular backfill. The tendon is 56
used to transmit the applied load to the base plate which compresses the granular 57
material to form the anchor. A study of the load–displacement response and ultimate 58
pullout capacity of granular anchors constructed in intact lodgement till and made 59
ground deposits is reported in this paper. Parallel tests were also performed on cast in-60
situ concrete anchors which are traditionally used for anchoring purposes. A new 61
method of analysis for the determination of the ultimate pullout capacity of granular 62
anchors is presented and verified experimentally, with the dominant mode of failure 63
controlled by the column length to diameter ratio. Granular anchors with L/D > 7 64
principally failed on bulging whereas short granular anchors failed on shaft resistance, 65
with the latter mobilising similar pullout capacities as conventional concrete anchors. 66
67
68
Key words: Ground improvement, anchors, retaining structures 69
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INTRODUCTION 70
Granular columns are traditionally used for improving weak deposits, and under 71
suitable conditions, offer a valuable means of increasing the bearing capacity of 72
foundations and stability of embankments founded on soft ground as well as reducing 73
total settlement and increasing the rate of consolidation. There has been some 74
discussion in recent years as to whether granular columns could also be used to resist 75
tension/pullout forces (Phani Kumar and Ramachandra Rao 2000, Liu et al. 2006, 76
Srirama Rao et al. 2007, Madhav et al. 2008, Phanikumar et al. 2008). Such granular 77
anchors consist of a horizontal base plate, a centrally-located tendon (stretched cable or 78
metallic rod) and compacted granular backfill. The tendon is used to transmit the 79
applied load to the column base via the circular base plate, which compresses the 80
granular material to form the anchor. The load can be applied to the anchor immediately 81
after its construction and drainage is also provided, via the granular column, to the soil 82
surrounding the anchor. Granular anchors have been used, for example, to prevent uplift 83
caused by flooding (Liu et al. 2006) and resist heaving of foundations in expansive 84
clays (Srirama Rao et al. 2007), and in such scenarios, have many applications for 85
lightly-loaded civil engineering structures, including residential buildings and 86
pavements. However, granular anchors can have much wider applications in the 87
construction industry, not only to enhance the stability of retaining structures, rock faces 88
or sheet piles but also to act as an effective drainage system in order to prevent 89
excessive build-up of pore water pressure, particularly in slope stabilization. However, 90
research is required to understand the load–displacement response, failure mode(s) and 91
ultimate pullout capacity of granular anchors, and importantly how they can be 92
appropriately integrated into routine civil engineering construction. This is the premise 93
that forms the basis to the research described in this paper. 94
95
EXPERIMENTAL PROGRAMME 96
The experimental studies reported in this paper were performed in three parts. The focus 97
of the first part was to compare the ultimate pullout capacity of granular anchors in 98
direct pullout against that of conventional cast in-situ concrete anchors. The ultimate 99
pullout capacity is the load at which the anchor is pulled out of the ground, either by 100
failure in shaft resistance mobilised between the granular/concrete column and 101
surrounding soil or alternatively, in the case of granular columns, by localised end-102
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bulging of the column itself (Hughes and Withers, 1974). These tests were performed at 103
Queen’s University Belfast (QUB), with the experimental programme considering the 104
assessment of two variables; namely anchor lengths (L) of 0.5, 1.0 and 1.5 m, and 105
anchor diameters (D) of 0.07 and 0.15 m. Incremental loading of the anchors in direct 106
tension was achieved using a custom-built loading device (Fig. 1) in which a bucket 107
supported on a loading arm of 3.0-m in overall length was progressively filled with 108
concrete cubes, each weighing ~64 N. The safe capacity of the loading bucket was 600 109
kg, which with a lever-arm ratio of 5:1, generated a possible maximum tension force of 110
~30 kN on the anchor tendon. The 1.2 × 0.75 m supporting platform spread the reaction 111
from the frame in order to reduce the bearing pressure on the supporting soil. 112
113
The second part of the investigation was performed at the Santry Sports Grounds of 114
Trinity College of Dublin (TCD) and examined in greater detail the performance of 115
granular anchors having different configurations, again considering the two variables of 116
L = 0.5, 1.0 and 1.5 m, with D = 0.15 and 0.20 m nominally. The tension/pullout 117
loading to the anchor tendon was applied using a hydraulic jack supported on a heavy 118
steel reaction frame (Fig. 2). The legs of the reaction frame were sufficiently distant 119
from the centrally-aligned tendon so as not to influence the anchor response. 120
121
The load–displacement response of the ground anchor system was measured using load 122
cells and long-stroke displacement transducers (see Figs. 1 and 2). The vertical 123
displacement of the ground surface was also measured at a distance of 0.3 m radially 124
from the anchor tendon by a displacement transducer mounted on an independent 125
reference beam (LVDT2 in Fig. 2). Load cells of 30 and 300 kN capacities were used to 126
measure the applied anchor load for the QUB and TCD tests, respectively, with the 127
mobilised load resistance recorded after a period of one minute following the 128
application of each load increment. 129
130
A single test was also performed at the TCD site in order to examine the viability of 131
using double anchor plates for the purpose of increasing the ultimate pullout capacity by 132
inducing bulging failure at two locations along the granular column. Due to constrains, 133
this aspect was not fully examined by means of full-scale field tests. Hence the third 134
part of the study involved performing laboratory model studies at QUB (Fig. 3), in 135
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which soft-firm stone-fee clay (undrained shear strength uc of ~30 kPa) was packed 136
into a wooden box of dimensions 1.2m×0.7m×0.7m in depth. Three column 137
configurations were examined: (a) L = 0.7 m and D = 0.035m, with a single plate 138
located at the bottom of the column; (b) L = 0.7 m and D = 0.035 m, with a plate 139
located at the bottom and a second plate located at mid height of the column; (c) L = 140
0.35 m and D = 0.035m,with a single plate located at the bottom of the column. Pull out 141
loading was applied using a pneumatic activator attached at the top of the reaction 142
frame (Fig. 3). 143
144
Ground conditions 145
The granular anchors at the QUB site were installed in made ground that had been 146
placed about 50 years previously, and was classified as firm to stiff clayey silty sand 147
with occasional gravel. Mean values of uc of 55 kPa were measured for depths greater 148
than 0.5 m below the ground surface, with slightly higher uc determined for shallow 149
depths. The in-situ bulk unit weight was 21kN/m3. Hand augurs with the relevant 150
diameters were used to bore holes in the ground in which the anchors were constructed. 151
Further details on the 5 tests (designated QUB1–5) performed on these granular anchors 152
are reported in Table 1. In addition, 4 tests were performed on concrete anchors. 153
154
All of the anchors at the TCD site were installed in the Upper Dublin Brown Boulder 155
Clay (UDBrBC) formation; a heavily-weathered stiff to very stiff, brown, slightly sandy 156
clay of low plasticity, with rare silt/gravel lenses. The geotechnical properties of the 157
Dublin Boulder Clay have been reported by Farrell et al. (1995) and Long and Menkiti 158
(2007), among others. Borehole logs for the TCD site indicated that the UDBrBC layer 159
was ~1.8 m in thickness across the test area, with mean values of water content of 12%, 160
bulk unit weight of 23 kN/m3 and a relatively high stone content (> 20 mm in particle 161
size) of between typically 5% and 10% measured over this depth. The standing 162
groundwater table was located at ~1.8–2.0 m below the ground surface, appearing to 163
approximately coincide with the transition boundary between the UDBrBC formation 164
and underlying Upper Dublin Black Boulder Clay formation. Larger bores of nominally 165
0.15 and 0.20 m in diameter were formed at this site by professional drillers using a 166
light cable-percussion drilling rig. Boreholes ~0.5 m in depth were formed using the 167
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clay cutter only, whereas deeper holes were formed using the clay cutter in combination 168
with a temporary steel casing, in accordance with British Standard BS879 (BSI, 1985). 169
Hence, with the casing removed, the actual bore diameter of the deeper holes was 170
equivalent to the outer casing diameter; i.e. precisely D = 0.168 and 0.219 m for holes 171
nominally 0.15 and 0.20 m in diameter. Further details on the 9 tests (designated 172
TCD1–9) performed on these granular anchors are reported in Table 2. 173
174
Anchor installation 175
Uniformly-graded basalt gravel (nominally 10-mm in size and with an angle of shearing 176
resistance g'φ of 45
o for the density achieved in the anchor setups) was used as backfill 177
for the QUB and TCD granular anchors and also as coarse aggregate in forming the 178
QUB concrete anchors. In the QUB laboratory model studies), the backfill material was 179
uniformly-graded basalt having particle sizes between 2.36 and 3.35 mm. In 180
constructing the anchors, the steel base plate with the tendon (threaded steel rod) was 181
inserted to the base of the borehole (Fig. 4a). The base plate diameters of 0.148 and 182
0.196 m used at the TCD site were marginally less than the diameters of the deeper 183
holes since a temporary casing had been required in forming the bore, which also had 184
the effect of producing a smooth borehole sidewall. In the case of the granular anchors, 185
the borehole was backfilled by pouring the gravel into the bore cavity to form ~0.12 m 186
thick layers, which were individually compacted to achieve maximum density using a 187
special hammer, comprising an annular compaction-plate and hollow tube assembly 188
(Fig. 4b), which fitted down around the anchor tendon. The mass of the hammer was 189
~2.5 kg and the gravel layers were compacted, in turn, by dropping the hammer 27 190
times through a free-fall distance of 0.7 m, which produced a bulk unit weight for the 191
gravel of 22 kN/m3. In the case of the concrete anchors, the bore cavity was backfilled 192
with a concrete mix prepared at a water-cement ratio of 0.55 in ~0.1 m layers which 193
were tamped using the same procedure used for the granular anchors. The concrete 194
anchors were allowed to cure for 7 days before performing the tension/pullout load 195
tests. 196
197
EXPERIMENTAL RESULTS 198
QUB Site 199
The experimental results of the first part of the study at the QUB test site, which 200
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compared the performance of granular and conventional concrete anchors, are reported 201
as tension load against vertical anchor displacement in Fig. 5. The pullout capacities of 202
the granular and concrete anchors of L x D = 0.5 x 0.07 m were 5.5 and 5.2 kN 203
respectively (Fig. 5a). The granular anchor displaced significantly (> 40 mm upward 204
movement of the top surface of the gravel column) during the course of loading 205
compared with the concrete anchor, although the displacement of the latter at the time 206
of failure was considerable (i.e. sudden pullout occurred), implying both of these 207
anchors failed on resistance mobilised along the column shaft. The soil surrounding the 208
concrete anchor did not undergo any significant displacement (either heave or 209
subsidence) until the failure state was achieved. However the soil surrounding the 210
granular anchor progressively heaved as the anchor was incrementally loaded to failure. 211
Anchors of L x D = 0.5 x 0.15 m also failed on shaft resistance (Fig. 5b), experiencing 212
ductile and sudden pullout behaviour for granular and concrete constructions, 213
respectively, with mobilised pullout capacities of 6.7 and 8.0 kN respectively. The 214
granular anchor of L x D = 1.0 x 0.07 m experiencing ductile failure, undergoing 215
localised end-bulging (Fig. 5c), whereas the concrete anchor experienced sudden 216
pullout, failing in shaft resistance. Pullout capacities of 16.1 and 16.3 kN were 217
mobilised for these granular and concrete columns respectively. During the early 218
loading stage, the surrounding ground barely moved, although ground heave started to 219
occur as the anchors approached pullout capacity. The 1.0 and 1.5 m long anchors of 220
0.15 m diameter (Fig. 5d) could not be taken to true failure since this exceeded the 221
capacity of the loading system used in performing these series of tests. Nevertheless, it 222
would appear from the load–displacement responses in Fig. 5d that failure of both 223
concrete and granular anchors was imminent at the time when the loading had to be 224
terminated prematurely, particularly in the case of the 1.0 m long anchors. 225
226
TCD Site 227
The experimental results of the second part of the study performed at the TCD test site 228
are shown in Fig. 6, including additional data of the vertical displacement response of 229
the surrounding ground measured at a distance of 0.3-m radially from the anchor 230
tendon. Short anchors of L x D = 0.45 x 0.148 m and 0.5 x 0.196 m (Fig 6a&b) failed 231
on shaft resistance, mobilising a pullout capacity of ~12 kN, with a visual observation 232
of the surface of the gravel backfill lifting in addition to substantial heave of the 233
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surrounding ground occurring once the applied load exceeded 10 kN. An increase in 234
anchor length and/or diameter produced an increase in pullout capacity. Anchors having 235
L = 0.96, 1.0 and 1.3 m with D = 0.219 m mobilised pullout capacities of 39, 42 and 44 236
kN respectively (Fig. 6a). In the case of 0.168 m diameter anchors, the pullout 237
capacities were 33, 40 and 42 kN for L = 0.8, 1.47 and 1.62 m respectively (Fig. 6b). A 238
marginal ground heave (~1 mm) was observed at failure in the case of 0.219-m diameter 239
anchors of L = 0.96 and 1.3 m (Fig. 6c). However, vertical displacements recorded at 240
the ground surface were insignificant (~0.2 mm) in the case of the 0.168-m diameter 241
anchors of L = 1.47 and 1.62 m (Fig. 6d), even though the anchors themselves had been 242
displaced by more than 100 mm. 243
244
The applied anchor load is resisted by the bulging capacity (Hughes and Withers, 1974) 245
of the granular column in the vicinity of the base plate and by shaft resistance mobilised 246
along the column shaft. Hence mobilisation of multiple bulging locations may 247
contribute to enhanced loading capacity. This possibility was examined in one of the 248
0.219-m diameter anchors (TCD9, Table 2) for which a second anchor plate was 249
positioned 0.7 m vertically above the base plate which was located at 1.4 m depth. The 250
relevant load–displacement curve is shown in Fig. 6a. The anchor resistance initially 251
plateau at ~40 kN, but a step increase in the load resistance subsequently occurred for 252
larger displacements (> 90 mm), followed shortly afterwards by pullout failure at an 253
anchor load of 44 kN. The fact that the pullout capacity mobilised by this 1.4-m long 254
double-plate anchor was less than that achieved by the 1.3-m long single-plate granular 255
anchor required further investigation and this will be covered later in the discussion 256
section. Also note that in one of the anchor tests, the load on the anchor was temporarily 257
removed and then re-applied (Fig. 6a), with the result that the unload–reload process 258
substantially increased the stiffness of the composite anchoring system. 259
260
DISCUSSION 261
Various methods of analyses that consider different failure modes (including vertical 262
slip, cone, circular arc) exist for the determination of the ultimate pullout capacity of 263
ground anchors constructed in homogeneous deposits of either sand or clay (Meyerhof 264
and Adams 1968, Ilamparuthi et al. 2002, Merifield and Sloan 2005). However, in the 265
case of granular anchors, the bore is backfilled with compacted granular material that is 266
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generally significantly different from surrounding native material. Under these 267
conditions, the failure mode can be complex and may involve localised bulging failure 268
at the base of the granular anchor (Hughes and Withers, 1974), mobilization of shaft 269
resistance and/or wedging failure, as illustrated in Fig. 7a. 270
271
In the full-scale studies performed at the QUB and TCD test sites, the granular anchors 272
generally failed at anchor displacements of ~60 mm. If the bulging mechanism was the 273
main cause of pullout failure, the enlargement in diameter occurring at the base of the 274
granular column may be ~10% of its original diameter at this anchor displacement, 275
assuming the length of bulging was twice that of the column diameter and no significant 276
movement of the gravel backfill occurred above the bulging zone. This localised and 277
marginal increase in column diameter may not be sufficient enough to trigger a wedging 278
failure mode. Hence, as a first approximation, only shaft resistance and localised end-279
bulging modes are considered in the following method of analysis proposed for granular 280
anchors. 281
282
The loading applied to the anchor tendon is simultaneously resisted by localised bulging 283
in the vicinity of the column base and by shaft resistance developed over the column 284
shaft (Fig. 7b), with the dominant failure mode governed by the column L/D ratio (see 285
later). In analogue to the ultimate pullout capacity of a rigid pile, the ultimate resistance 286
of the granular anchor in shaft resistance, including its self-weight contribution, is given 287
by 288
289
4
2g
uF
LDcDLT
γπαπ += Equation 1 290
291
where L and D are anchor length and diameter respectively; uc is the undrained shear 292
strength of the surrounding soil; α an adhesion factor and γg is the unit weight of the 293
granular backfill. 294
295
Note that vacuum cannot develop in the cavity that forms directly below the base plate 296
during pullout on account of the open pore structure of the granular column. The local 297
bulging capacity of the granular column itself is given by 298
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299
4
2v
B
DT
σπ= Equation 2a 300
301
with the bearing pressure at the column base vσ estimated using the relationship 302
proposed by Hughes and Withers (1974): 303
304
[ ]ucvc
g
g
v cN *
'sin1
'sin1+
−
+= σ
φ
φσ Equation 2b 305
306
where vcσ is the overburden pressure caused by the surrounding soil at the point of 307
bulging; g'φ is the angle of shearing resistance of the granular column and *
cN is a 308
bearing capacity factor that considers local shear failure. Gibson and Anderson (1961) 309
proposed that the value of *
cN was given by 310
311
u
cc
GN log1* += Equation 3 312
where G is the shear modulus of the soil. 313
314
In the present investigation, ucG = 100 was assumed for the TCD site, and 315
accordingly *
cN = 4.6. The undrained shear strength against depth profile of the 316
surrounding soil is the crucial piece of information required for the prediction of anchor 317
performance/mode of failure, with shaft resistance mobilised along the full length of the 318
column shaft whereas bulging occurs locally in the vicinity of the column base. 319
320
QUB Site 321
The experimental programme at the QUB site considered the assessment of anchor 322
length (0.5, 1.0 and 1.5 m) and diameter (0.07 and 0.15 m) on ultimate pullout capacity. 323
Control tests were also performed using concrete anchors of similar dimensions. The 324
strength against depth profile of the ground determined using a hand vane indicated an 325
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average undrained strength of 55 kPa for depths greater than 0.5-m below the ground 326
surface (Fig. 8). 327
328
In granular column applications for ground improvement, the column can fail by one of 329
two distinct mechanisms. As the load increases on the granular column, the shaft 330
resistance developed along the cylindrical surface and the end bearing resistance 331
developed at the base of the granular column are mobilised gradually. This is typical for 332
short columns and for values of L/D ratio < ~6–7 (Black et. al. 2011, Sivakumar et. al. 333
2011, Wood et. al. 2000, Hughes and Withers, 1975). In contrast, longer columns fail in 334
localised bulging occurs in the vicinity of the column head since the shaft resistance and 335
end bearing capacities exceed the bulging capacity. This analogy can be extended to 336
granular anchors, with the proviso that bulging in granular anchors occurs close to the 337
bottom of the column. 338
339
Failure over the column length would occur due to a shear zone developing within the 340
remoulded soil next to the bore sidewall and not along the granular/soil interface since 341
no distinct granular surface forms, with the confined granular material intruding slightly 342
into the adjacent soil under pullout loading. Hence α = 1 is assumed in determining the 343
shaft resistance. This is also supported by back-calculating the value of α from the 344
observed performance of the concrete anchors. Table 1 lists values of predicted shaft 345
resistance and bulging capacities together with measured pullout loads at the 346
termination of each test. Note that loading was terminated at 30 kN load for one of the 347
anchors (QUB5) on account of the load cell capacity being reached. Based on available 348
information, it can be concluded that the 0.07 and 0.15-m diameter by 0.5-m long 349
anchors failed in shaft resistance whereas the 0.07 and 0.15-m diameter by 1.0-m long 350
anchors may have failed by localised end-bulging. This postulation is further illustrated 351
by plotting bearing pressure acting on the column base against L/D ratio (see Fig. 9). 352
Included in this figure and indicated by a broken line, is the mobilisation of shaft 353
resistance for an average undrained shear strength of 55 kPa over the column length. 354
Based on the results obtained, it can be concluded that the 0.07 and 0.15-m diameter by 355
0.5-m long anchors failed on shaft resistance whereas the other anchors may have failed 356
on bulging. Furthermore the work clearly suggests that the L/D ratio which 357
distinguishes whether pullout failure occurs in shaft resistance or localised end-bulging 358
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is about 7. The results from the QUB model studies on double-plate capacity will be 359
discussed later in this paper for clarity reasons. 360
361
TCD Site 362
Figure 10 shows the undrained strength against depth profile for the TCD site. In situ 363
probing and laboratory strength measurements were made using a 20-tonne CPT truck 364
and unconsolidated-undrained triaxial compression tests performed on 100-mm 365
diameter by 200-mm high specimens reconstituted by standard Proctor-compaction of 366
material at its natural water content that had been recovered using the clay cutter during 367
borehole formation. A few ‘undisturbed’ specimens that had been obtained from just 368
below the base of the boreholes using a 38-mm diameter sampling tube were also tested 369
in triaxial compression. The CPT-derived peak undrained shear strength 370
( ) ktvocu Nqc σ−= , where cq is the CPT cone-tip resistance and voσ is the overburden 371
pressure. However no major study of this relationship has been reported in literature for 372
Dublin Boulder Clay, mainly because of limited penetrations achieved, and the cq 373
profile also tends to be ‘spiky’ due to the presence of stones and inherent variability of 374
the material. This was collaborated by significantly higher gravel contents observed at 375
certain levels within recovered borehole cores. Hence an average value of ktN = 15, 376
given by Lunne et al. (2002) for lodgement till deposits, was deemed appropriate. 377
Unsurprisingly the CPT peak uc was consistently greater than laboratory measurements 378
on reconstituted specimens. However, since granular anchors are generally taken 379
through large displacement and interaction between the granular material and 380
surrounding soil is more intense than may prevail in the case of rigid piles, strength 381
parameters obtained from remoulded test-specimens are considered appropriate in this 382
analysis. An average uc = 55 kPa was used for depths of up to 0.8 m below the ground 383
surface, with a step increase to uc = 80 kPa assumed for greater depths (Fig. 10). 384
385
Predicted anchor loads based on failure in shaft resistance and localised end-bulging 386
modes (Eqs. 1 and 2 respectively) are listed in Table 2. Note that ultimate pullout 387
capacity by failure in shaft resistance increases linearly, and is strongly sensitive to, 388
increasing L/D ratio. Bulging capacity depends on ucG , g'φ and L/D ratio on account 389
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13
of the increase in confining stress and undrained strength with depth, although for a 390
given column diameter, the experimental pullout capacity by bulging failure was found 391
to increase only marginally with increasing L/D ratio (see Table 2). Shaft failure is 392
generally dominant in short columns whereas bulging failure can be expected in longer 393
columns. In the case of the 0.168-m diameter anchors, the longer columns with L = 1.47 394
and 1.62 m failed on bulging (L/D = 8.8 and 9.6 respectively). Furthermore the ground 395
heave measured for these anchors was insignificant (Fig. 6c), validating the argument 396
for bulging failure having occurred in the vicinity of the column base. Similar diameter 397
columns but with L = 0.45 and 0.80 m failed on mobilazation of shaft resistance, 398
although the pullout capacity for the latter was noticeably higher than the predicted 399
shaft resistance. This may be due to some variability in strength due to heterogeneity of 400
the lodgement till material at the location of the testing. The occurrence of shaft failure 401
was further substantiated in the case of the 0.45-m long column, which underwent 402
significant ground heave from the start of loading (Fig. 6d), and also indicates good 403
interaction between the granular column and surrounding soil. It appears that none of 404
the 0.219-m diameter columns failed on bulging, with the measured pullout capacity in 405
close agreement with the predicted capacity in shaft resistance. 406
407
The bearing pressure acting on the column base for the TCD anchors was determined 408
using Eq. (2b) and is plotted against the column L/D ratio in Fig. 11. Included also is 409
the predicted capacity in shaft resistance based on the measured remoulded uc value of 410
55 kPa. Based on the results obtained, it can be concluded that the two 0.168-m 411
diameter anchors with L/D > 7 (i.e. TCD 3 and 4) failed on bulging. However in the 412
case of the 0.219-m diameter anchors, it is possible that failure in both bulging and shaft 413
resistance may have occurred simultaneously, at least for the longer columns. 414
415
As reported earlier, the TCD double-plate granular anchor system (L = 1.4m with the 416
second plate firmly located at mid height, i.e. 0.7 m depth) exhibited some complex 417
behaviour (Fig. 6a), with its measured overall pullout capacity lower than that of the 418
1.3-m long anchor with a single plate located at the base. The differences in 419
performance can be explained by considering the mode of failure developed for the two 420
segments of the double-plate anchor system. Figure 12 illustrates single and double-421
plate granular anchor system configurations. The bottom plate in the single plate system 422
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14
(Fig. 12a) and bottom- and mid-plates in the double-plate system (Fig. 12b) move 423
vertically as the pullout loading is applied. In the latter case, assuming insignificant 424
extension of the steel tendon under loading, vertical displacements of the bottom- and 425
mid-plates are similar and cavities may also develop directly below the plates (see Fig. 426
12c). Note that a cavity may also develop for the single-plate anchor system. This 427
therefore suggests that mobilisation of bulging resistance and shaft resistance are 428
identical at the both segments of the double-plate anchor system, assuming a uniform 429
undrained shear strength profile. This implies that for the double-plate configuration, 430
the two segments of the anchor system behave as independent units, hence the 431
responses are also practically independent and controlled by the values of L/D ratio for 432
the respective segments. Compared with L/D = ~5.9 for the 0.219-m diameter by 1.3-m 433
long single-plate system tested at the TCD site, the L/D ratio for the two equal segments 434
of the double-plate anchor system was 3.2 , considerably less than the critical L/D ratio 435
of ~ 7 required for potential bulging failure. Hence, for a uniform undrained strength 436
against depth profile, the resistances mobilised by the equal-length segments of the 437
experimental double-plate anchor system will be similar. Moreover, the two segments 438
of the double-plate anchor behave as independent units. Hence their individual 439
responses are largely controlled by the values of L/D ratio for the respective segments. 440
Based on this postulation, a simple estimation for this shaft resistance was calculated 441
based on the measured pullout capacity of the 0.196-m diameter by 0.5-m long anchor 442
which failed on shaft resistance at the TCD site (see Fig. 6). The estimations involved 443
taking account of the different diameters and lengths of these anchors. Figure 13 shows 444
the actual performance of the double-plate anchor system and predicted performance 445
based on these calculations. The agreement is good, though the authors agree that it is 446
only an approximation. This intriguing response of the double-plate anchor system 447
prompted further investigation by the authors using model studies. . 448
449
Three model anchors having the same diameter of 0.035 m but (a) 0.7 m long with 450
single base plate (i.e. L/D = 20); (b) 0.7 m long with double-plate (L/D = 10 for each 451
segment) and (c) 0.35 m long with single base plate (i.e. L/D = 10) were constructed in 452
a soft-firm stone-free clay bed (Fig. 3). The relevant load–displacement characteristics 453
are shown in Fig. 14. The 0.35 and 0.70-m long anchors having a single plate located at 454
the bottom of the column failed at pullout capacities of about 575 and 650 N 455
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15
respectively. These two observations are generally similar. However in the case of 456
double-anchor system, the failure load of 1350 N was significantly greater, at least 457
double that measured for the 0.7-m long anchor having a single plate at the bottom. In 458
all three cases, values of L/D ratio were greater than 10 suggesting a potential bulging 459
failure. This therefore confirms that the pullout capacity can be enhanced by employing 460
double or multiple plate anchor systems provided that the L/D ratio of each segment is 461
higher than the critical L/D ratio of ~ 7. 462
463
Finally, granular anchors performed to similar pullout capacities as concrete anchors 464
tested at the QUB site, suggesting that granular anchors might provide an alternative 465
option to concrete anchors in future engineering construction but does this not only 466
apply for columns with L/D < 7. However it is important to note that granular anchors 467
undergo significant displacements in mobilising ultimate pullout capacity whereas 468
concrete anchors generally failed at very low displacements (Fig. 5). While 469
displacement is not a favourable outcome of any geotechnical or engineering 470
application, progressive displacement of the granular anchor under loading (ultimately 471
resulting in ductile failure) can be considered as an early warning of possible failure of 472
the anchoring system occurring, as opposed to more sudden/brittle failure in the case of 473
concrete anchors. 474
475
Bulging failure can be restricted by enclosing the granular column in geotextile 476
(Sivakumar et al.,2000), and in this case, the column will also partially utilise potential 477
shaft resistance available under pullout loading. However it should be noted that such 478
an inclusion of geotextile may hinder the interaction between granular column and 479
surrounding soil, thereby potentially mobilising reducing shaft resistance, although 480
further research is necessary. 481
482
483
CONCLUSIONS 484
485
This paper has presented the construction, testing and performance of granular anchors 486
in old filled deposits (QUB site) and an intact lodgement till deposit (TCD site). 487
Granular anchors of different L/D ratio were loaded to failure in direct tension/pullout. 488
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Granular anchors of larger surface area, achieved by increasing the anchor length and/or 489
diameter, mobilised greater ultimate pullout capacity of up to ~45 kN at TCD site. 490
491
A new method of analysis for the determination of the ultimate pullout capacity has 492
been presented and verified experimentally. The applied anchor load is simultaneously 493
resisted by localised bulging in the vicinity of the column base and by shaft resistance 494
mobilised along the length of the column, with the dominant failure mode governed by 495
the column L/D ratio. Granular anchors having L/D > 7 principally fail on bulging and 496
are particularly effective in transferring applied loads to strata at depth. The study has 497
also demonstrated that the pullout capacity can be increased significantly using a 498
multiple-plate anchor system, provided the L/D ratio of individual column segments is 499
greater than the critical value. 500
501
Granular anchors are a good alternative to traditional anchoring methods. Short granular 502
anchors principally fail on shaft resistance and were found to mobilise similar pullout 503
capacity compared with conventional cast in-situ concrete anchors. Other advantages of 504
granular anchors include short construction time, lower costs as well as the ability of 505
resist applied loading immediately after construction. Granular anchors displace 506
significantly under increasing applied load (with pullout failure generally occurring for 507
anchor displacements of ~60 mm in the present study) compared with the more rigid–508
perfectly plastic (i.e. sudden pullout) response of concrete anchors. However, a 509
significantly stiffer response can be achieved for granular anchors by simply performing 510
a single unload–reload cycle. 511
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17
Acknowledgements
The authors would like to thank Martin Carney and Eoin Dunne for their assistance in
performing the anchor tests at the Santry test site.
References
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Meyerhof, G.G., and Adams, J.I. (1968) The ultimate uplift capacity of foundations.
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foundations: model tests. Géotechnique, Vol. 50, No. 6, 689–698.
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granular pile anchors in expansive soils using base geosynthetics. Canadian
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Table 1 Predicted shaft resistance and bulging capacities and measured pull out loads of
granular anchors at QUB test site. Note: F and B, failure in shaft resistance and end-
bulging respectively.
*Test terminated without mobilising ultimate pullout capacity
Table 2 Predicted shaft resistance and bulging capacities and measured pull out loads of
granular anchors at TCD test site. Note: F and B, failure in shaft resistance and end-
bulging respectively.
Test
no. Bore
diameter
(m)
Diameter
base plate
(m)
Column
length
L/D
Shaft
capacity Bulging
capacity
Measured
pullout
capacity
Mode
of
failure
(m)
ratio
kN
kN kN
TCD1 0.148 0.148 0.45 3.0 12 26 12 F
TCD2 0.168 0.148 0.80 4.8 23 27 33 F
TCD3 0.168 0.148 1.47 8.8 43 40 40 B
TCD4 0.168 0.148 1.62 9.6 47 40 42 B
TCD5 0.196 0.196 0.50 2.6 17 46 12 F
TCD6 0.219 0.196 0.96 4.4 36 68 39 F
TCD7 0.219 0.196 1.00 4.6 37 68 42 F
TCD8 0.219 0.196 1.30 5.9 49 70 45 F
TCD9* 0.219 0.196 1.40 6.4 53 69 44 F
* Double-plate anchor with mid-height plate located at 0.7 m below the ground
surface.
Test
no. Bore
diameter
(m)
. Diameter
base plate
(m)
Column
length
(m)
L/D
ratio
Shaft
capacity
kN
Bulging
capacity
kN
Measured
pullout
capacity
kN
Mode
of
failure
QUB1 0.07 0.07 0.5 7.0 6.1 5.9 5.2 F
QUB2 0.07 0.07 1.0 14.0 12.2 6.1 16.5 B
QUB3 0.15 0.15 0.5 3.3 13.2 27.0 7.5 F
QUB4 0.15 0.15 1.0 6.7 26.3 28.1 30.7 F/B
QUB5 0.15 0.15 1.5 10.0 39.4 29.1 30.8* B
Page 19 of 34C
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Figure Captions
Figure 1. Schematic of loading frame; QUB study (not to scale)
Figure 2. Schematic of loading frame; TCD study (not to scale)
Figure 3. Testing set-up (model study at QUB)
Figure 4. Granular anchor
Figure 5. Load-displacement characteristics of concrete and granular anchors (QUB Site)
Figure 6. Load-displacement characteristics of granular anchors (TCD Site)
Figure 7. Failure mechanisms
Figure 8. Undrained shear strength profile (QUB site)
Figure 9. Bearing pressure vs L/D ratio QUB site
Figure 10. Strength profile (TCD Site)
Figure 11. Bearing pressure vs L/d ratio TCD site
Figure 12. Failure mechanisms (double plate)
Figure 13. Load-displacement characteristics, single plate (0.5m and double plate 1.4m)
Figure 14. Load-displacement characteristics, single plate and double plate
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Figure 1 Schematic of loading frame; QUB study (not to scale)
Loading bucket
Granular column
Base plate
Steel rod
LVDT 1
LVDT 2
Load cell
Support
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w.n
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chpr
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by
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nity
Col
lege
Dub
lin o
n 12
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12Fo
r pe
rson
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se o
nly.
Thi
s Ju
st-I
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anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
Granular column
Base plate
Steel rod
LVDT 1 LVDT 2
Load cell
Supports
Support frame
To hydraulic jack
Figure 2 Schematic of loading frame; TCD study (not to scale)
Page 22 of 34C
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nity
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lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
Figure 3 Testing set-up (model study at QUB)
Proving ring
Anchor rods
Pressure supply to
pneumatic activator
Test box: 1.2mX0.7mX0.7m
(Depth)
Page 23 of 34C
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
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r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
Figure 4 Granular anchor
Ground surface
Steel rod
Borehole
Base plate
Granular backfill
Compaction plate
Hollow tube
(a) Column construction (b) Compactor
Page 24 of 34C
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
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ial v
ersi
on o
f re
cord
.
Figure 5 Load-displacement characteristics of concrete and granular anchors (QUB Site)
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
5 10 15 20 25 30 35 40
Displacement (mm)
Load (
kN
)
Granular anchor
Concrete anchor
0
1.0
2.0
3.0
4.0
5.0
6.0
0 10 20 30 40 50 Displacement (mm)
Load (
kN
)
Granular anchor
Concrete anchor
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
0 20 40 60 80 100 120
Displacement (mm)
Load (
kN
)
Granular anchor
Concrete anchor
0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
0 10 20 30 40 50 60 70
Displacement (mm)
Load (
kN
)
Concrete anchor (1 m long, test
stopped)
Granular anchor 1.5m long (test
stopped)
Granular anchor 1.0m long (test
stopped)
(c) 1m long 0.07m diameter column (d) 1&1.5m long 0.150m diameter column
(a) 0.5m long 0.07m diameter column (b ) 0.5m long 0.150m diameter column
Page 25 of 34C
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by
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
0
5
10
15
20
25
30
35
40
45
50
0 50 100 150
0
2
4
6
8
10
12
0 50 100 150
0
5
10
15
20
25
30
35
40
45
0 50 100 150
0
5
10
15
20
25
0 50 100 150
0.148m f , 0.45m long 0.196m f , 0.5m long
0.196m f , 0.5m long
0.219m f , 1.3m long
0.148m f , 0.45m long
Unloading-reloading
0.219m f , 0.96m long
0.219m f , 1.4m long
(double plate)
0.219m f , 1.0m long
0.219m f , 0.96m long
0.219m f , 1.3m long
0.168m f , 1.62m long
0.168m f , 0.8m long
0.168m f , 1.47m long
0.168m f , 0.8m long
0.168m f , 1.47&1.62m long
Anchor displacement (mm)
Load (
kN
)
Load (
kN
) D
ispla
cem
ent a
dja
cent
to a
nchor
mm
Anchor displacement (mm)
Anchor displacement (mm) Anchor displacement (mm)
Figure 6 Load-displacement characteristics of granular anchors (TCD Site)
(a) 0.219 m diameter anchor
Dis
pla
cem
ent a
dja
cent
to a
nchor
mm
(b) 0.168m diameter anchor
(c) 0.219 m diameter (displacement away from anchor) (c) 0.168m diameter (displacement away from anchor)
Page 26 of 34C
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nity
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lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
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r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
Do
Df
Friction mobilization
Diameter after bulging
Bulging
Original diameter
Wedge formation Shaft resistance
Bulging region
Column length L
Column diameter D
Bulging resistance
Figure 7 Failure mechanisms
(a) Possible failure mechanisms (b) Bulging and shaft resistance
Page 27 of 34C
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by
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
Figure 8 Undrained shear strength profile (QUB site)
Undrained strength cu kPa
10 20 30 40 50 80 70 60
0
0.2
0.4
0.6
0.8
1.2
1.0
1.4
Depth
m
General trend
Average undrained shear
strength below 0.5m
Page 28 of 34C
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
Figure 9 Bearing pressure vs L/D ratio QUB site
0
1.0
2.0
3.0
4.0
0.0 4.0 8.0 12.0 16.0
L/D Ratio
Bearin
g P
ressure
(M
Pa)
0.07m f, 0.5m long (L/D =7
0.15m f, 1.0m long (L/D =6.7)
0.15m f, 0.5m long (L/D =3.3)
0.07m f, 1.0m long (L/D =14)
0.15m f, 1.5m long (L/D =10.0)
Mobilisation of shaft
resistance
Page 29 of 34C
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 100 200 300 400
Depth
, m
bgl
Undrainerd shear strength, kPa
DBrBC_38mm undisturbed
DBrBC_100mm remoulded
DBkBC_100mm remoulded
uCPT (undisturbed)
Figure 10 Strength profile (TCD Site)
Page 30 of 34C
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nity
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lege
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lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
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r fr
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nal o
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ial v
ersi
on o
f re
cord
.
Figure 11 Bearing pressure vs L/d ratio TCD site
L/D Ratio
Bearin
g P
ressure
(M
Pa)
Friction failure line
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 2.0 4.0 6.0 8.0 10.0 12.0
Plate diameter 0.148 m
Plate diameter 0.196 m
Page 31 of 34C
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
Friction mobilization
Bottom Plate
Bulging
Friction mobilization
Bottom plate
Mild bulging
(a) Single plate, at failure (1.0m long) (b) Double plate, initial conditions
(First plate at 0.7m and second plate at 1.4m)
(c) Double plate, failure conditions
(First plate at 0.7m and second plate at 1.4m)
Cavity below first plate
Cavity below second plate
Mild bulging
Middle plate
Friction mobilization
Figure 12 Failure mechanisms (double plate)
Page 32 of 34C
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
0.0 20.0 40.0 60.0 80.0 100.0 120.0
0.196m f , 0.5m long
Predicted performance based on
0.196m f , 0.5m long
0.219m f , 1.4m long
(double plate)
Anchor displacement (mm)
Load (
kN
)
Figure 13 Load-displacement characteristics, single plate (0.5m and double plate 1.4m)
Page 33 of 34C
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by
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
al u
se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
iffe
r fr
om th
e fi
nal o
ffic
ial v
ersi
on o
f re
cord
.
Figure 14 Load-displacement characteristics, single plate and double plate
0
200
400
600
800
1000
1200
1400
0 10 20 30 40
0.035m f , 0.7m long
(double plate)
0.035m f , 0.7m long
(single plate)
0.035m f , 0.35m long
(single plate)
Anchor displacement (mm)
Load
N
Page 34 of 34C
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nity
Col
lege
Dub
lin o
n 12
/14/
12Fo
r pe
rson
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se o
nly.
Thi
s Ju
st-I
N m
anus
crip
t is
the
acce
pted
man
uscr
ipt p
rior
to c
opy
editi
ng a
nd p
age
com
posi
tion.
It m
ay d
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r fr
om th
e fi
nal o
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ial v
ersi
on o
f re
cord
.